The prior art has disclosed a particle beam device, for example a scanning electron microscope or an ion beam device, which is provided with a sample stage, on which an object to be examined and/or to be processed is arranged. The sample stage has a movable design, wherein the movable design of the sample stage is ensured by a plurality of movement elements, from which the sample stage is composed. The movement elements enable a movement of the sample stage in at least one specific direction. In particular, sample stages are known which have a plurality of translational movement elements (for example approximately 3-4 translational movement elements) and a plurality of rotational movement elements (for example 2-3 rotational movement elements). By way of example, a sample stage is movably arranged along a first translation axis (for example an x-axis), along a second translation axis (for example a y-axis) and along a third translation axis (for example a z-axis). The first translation axis, the second translation axis and the third translation axis are oriented perpendicular to one another. Furthermore, the sample stage is rotatably arranged about a first rotation axis and about a second rotation axis, which is perpendicular to the first rotation axis.
The driving force for a movement by means of the movement elements is provided by stepper motors in the prior art. Respectively one stepper motor is provided for respectively one movement along one of the translation axes or for one rotation about one of the rotation axes. Here, the stepper motors can be arranged within a vacuum chamber of a particle beam device, or else be arranged outside of the vacuum chamber of the particle beam device. In the latter case, corresponding vacuum feed-throughs and mechanical apparatuses are provided for ensuring the actuation between the stepper motors and the sample stage.
A stepper motor known from the prior art has the following basic design. The stepper motor is provided with a rotor, which is rotatably arranged in the stepper motor. Furthermore, the stepper motor has coils, which are arranged about the rotor. The coils provide a controlled, incrementally rotating electromagnetic field, by means of which the rotor can be rotated about a minimal angle or about a multiple of this minimal angle. This renders it possible to obtain a certain number of steps per rotation of the rotor. The prior art has disclosed stepper motors with different numbers of steps per rotation of the rotor, for example with 100 steps per rotation. In the case of a stepper motor with 100 steps per rotation, the rotor in each case rotates by 3.6 degrees in one full step.
A so-called micro-step operation is also possible in known stepper motors in addition to an operation in which the stepper motor carries out full steps. In the micro-step operation, smaller steps than a full step are provided. To this end, the step angle is reduced. This is provided by an actuation of the motor current for the coils used in the stepper motor. By switching the motor current on or off at the individual coils of the stepper motor, a step-like overall control current profile is obtained, which is provided by the relationships of the amplitude of the motor current applied to the individual coils. Hence, the stepper motor is actuated by a selectable actuation of the motor current for the individual coils used in the stepper motor. As a result of a selectable phase-shifted actuation of the motor current for the individual coils of the stepper motor, it is possible to obtain full steps or smaller steps (for example half steps, eighth steps or smaller steps). The amplitude of the first motor current, which is applied to a first coil of a stepper motor, and the amplitude of a second motor current, which is applied to a second coil of the stepper motor, do not depend on the step size (i.e., for example, a full step, a half step or any other step) and are therefore always the same for each selected step width.
It is desirable in a particle beam device for the positional setting of the sample stage to be as accurate as possible. This is desirable, in particular for a high resolution or for accurate imaging of an object (sample) arranged on the sample stage. If the micro-step operation is now used in a stepper motor, this can lead to problems in setting the position of the sample stage when the stepper motor is stopped. For stopping purposes, the rotor of the stepper motor is decelerated. This means that, when the sample stage is decelerated (and accordingly also when the sample stage is stopped), the frequency of the motor current for actuating the individual coils is reduced to zero. The frequency of the motor current for the stepper motor is a measure for the speed of the stepper motor, and hence also a measure for the speed of the sample stage. When the stepper motor is stopped, the rotor stops in a predeterminable position, which is predetermined by one of the micro-steps. In this position, the motor current assumes a value, which is predetermined by an operational amplitude and an operational phase of the motor current, for the stepper motor, required in this motor position. This operational amplitude is static. It can lead to an excessive thermal load on the stepper motor, which is undesirable. It is for this reason that provision is made in the prior art for the amplitude of the motor current to be reduced to a predeterminable holding amplitude after stopping. The motor current having this holding amplitude is also referred to as a holding current. In the case of the holding current, the heating of the components is at acceptable levels.
In respect of the prior art, reference is made to DE 10 2009 028 600 A1 and DE 10 2010 020 550 A1.
However, as a result of the existing electromagnetic fields, the rotor of the stepper motor moves slightly in the direction of the next full step when the motor current is reduced to the predeterminable holding amplitude. This also moves the sample stage. Accordingly, this leads to an additional, undesired movement and to an undesired position of the sample stage. The object arranged on the sample stage may be positioned incorrectly.
Accordingly, it is desirable, firstly, for the position of the sample stage to be able to be driven at accurately and without undesired additional movement and, secondly, for the position of the sample stage always to be known with great accuracy. Thus, it would be desirable to specify a method by means of which this can be achieved.